That just sounds like an argument from idealism. Writing software is all about tradeoffs, and not supporting non-Linux systems is one tradeoff that systemd make

That just sounds like an argument from idealism. Writing software is all about tradeoffs, and not supporting non-Linux systems is one tradeoff that systemd makes, for a good reason. In this case, close integration with the kernel is required to be able to produce the features envisioned. You might perhaps find some instances where systemd components are needlessly interrelated, but the claim that it isn't modular is mostly a myth.